Downhole tools, such as wireline tools, bottom hole assemblies attached to a drill string, each having a variety of sensors are commonly utilized to determine a variety of parameters of interest relating the subsurface formations, including detection of formation fluids flowing into the wellbores. It is useful to detect the presence, extent, and location (depth) of the wellbore fluids entering a wellbore. Such information may be utilized for completing the wells, performing remedial work and/or to determine one or more characteristics of the reservoir or the formation.
An increase in the demand of natural gas has led to the need to complete low volume gas wells. This demand has caused the oil and gas exploration industry to identify small, low volume gas entering into the wellbores. These wells may be air drilled boreholes or drilled utilizing drilling fluid. It is thus useful to detect the presence and location of such small gas producing zones.
Also, of importance is the detection of liquids into the wellbores whether prior to completing such wells for producing hydrocarbons or after completion. It is useful to detect whether a formation fluid is leaking into a wellbore after completion for remedial work. Acoustic sensors, including ultrasonic sensors, carried by downhole tools have been utilized to detect formation fluid flows into the wellbores. In certain downhole situations, fluid entering the wellbores through small areas create turbulences in the wellbore fluid (which may be liquid or air) in the ultrasound frequency range. Ultrasonic sensors have been utilized to detect such turbulence. Ultrasonic sensors utilizing a piezoelectric element have been utilized. Such sensors are enclosed in an outer casing which may be a metallic or a non-metallic (plastic, rubber, etc.) housing to protect the sensor from the borehole environment (high pressure and temperature). The protective encasement tends to reduce the ability of the sensor to detect the ultrasonic turbulence due to the sound reflection and/or attenuation due to the protective casing material.
Thus, it is desirable to have downhole tools that include ultrasonic sensors that have greater sensitivity and which are able to withstand the downhole environment, i.e., high temperatures and high pressures (which may be greater that 300 degrees Fahrenheit and over 20,000 psi). The present invention provides an apparatus and method that address the above-noted problems.